NAME

DESCRIPTION

The section of the FAQ answers question related to the manipulation of data as numbers, dates, strings, arrays, hashes, and miscellaneous data issues.

Data: Numbers

Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?

The infinite set that a mathematician thinks of as the real numbers can only be approximate on a computer, since the computer only has a finite number of bits to store an infinite number of, um, numbers.

Internally, your computer represents floating-point numbers in binary. Floating-point numbers read in from a file or appearing as literals in your program are converted from their decimal floating-point representation (eg, 19.95) to the internal binary representation.

However, 19.95 can't be precisely represented as a binary floating-point number, just like 1/3 can't be exactly represented as a decimal floating-point number. The computer's binary representation of 19.95, therefore, isn't exactly 19.95.

When a floating-point number gets printed, the binary floating-point representation is converted back to decimal. These decimal numbers are displayed in either the format you specify with printf(), or the current output format for numbers (see "$#" in perlvar if you use print. $# has a different default value in Perl5 than it did in Perl4. Changing $# yourself is deprecated.

This affects all computer languages that represent decimal floating-point numbers in binary, not just Perl. Perl provides arbitrary-precision decimal numbers with the Math::BigFloat module (part of the standard Perl distribution), but mathematical operations are consequently slower.

Why isn't my octal data interpreted correctly?

Perl only understands octal and hex numbers as such when they occur as literals in your program. If they are read in from somewhere and assigned, no automatic conversion takes place. You must explicitly use oct() or hex() if you want the values converted. oct() interprets both hex ("0x350") numbers and octal ones ("0350" or even without the leading "0", like "377"), while hex() only converts hexadecimal ones, with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".

This problem shows up most often when people try using chmod(), mkdir(), umask(), or sysopen(), which all want permissions in octal.

Does Perl have a round() function? What about ceil() and floor()? Trig functions?

Remember that int() merely truncates toward 0. For rounding to a certain number of digits, sprintf() or printf() is usually the easiest route.

printf("%.3f", 3.1415926535); # prints 3.142

The POSIX module (part of the standard perl distribution) implements ceil(), floor(), and a number of other mathematical and trigonometric functions.

use POSIX;
$ceil = ceil(3.5); # 4
$floor = floor(3.5); # 3

In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex module. With 5.004, the Math::Trig module (part of the standard perl distribution) implements the trigonometric functions. Internally it uses the Math::Complex module and some functions can break out from the real axis into the complex plane, for example the inverse sine of 2.

Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself.

To see why, notice how you'll still have an issue on half-way-point alternation:

Don't blame Perl. It's the same as in C. IEEE says we have to do this. Perl numbers whose absolute values are integers under 2**31 (on 32 bit machines) will work pretty much like mathematical integers. Other numbers are not guaranteed.

How do I convert bits into ints?

To turn a string of 1s and 0s like 10110110 into a scalar containing its binary value, use the pack() function (documented in "pack" in perlfunc):

$decimal = pack('B8', '10110110');

Here's an example of going the other way:

$binary_string = join('', unpack('B*', "\x29"));

Why doesn't & work the way I want it to?

The behavior of binary arithmetic operators depends on whether they're used on numbers or strings. The operators treat a string as a series of bits and work with that (the string "3" is the bit pattern 00110011). The operators work with the binary form of a number (the number 3 is treated as the bit pattern 00000011).

How can I output Roman numerals?

Get the http://www.perl.com/CPAN/modules/by-module/Roman module.

Why aren't my random numbers random?

If you're using a version of Perl before 5.004, you must call srand once at the start of your program to seed the random number generator. 5.004 and later automatically call srand at the beginning. Don't call srand more than once--you make your numbers less random, rather than more.

Computers are good at being predictable and bad at being random (despite appearances caused by bugs in your programs :-). http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom Phoenix, talks more about this.. John von Neumann said, ``Anyone who attempts to generate random numbers by deterministic means is, of course, living in a state of sin.''

If you want numbers that are more random than rand with srand provides, you should also check out the Math::TrulyRandom module from CPAN. It uses the imperfections in your system's timer to generate random numbers, but this takes quite a while. If you want a better pseudorandom generator than comes with your operating system, look at ``Numerical Recipes in C'' at http://www.nr.com/ .

Of course, this believes that weeks start at zero. The Date::Calc module from CPAN has a lot of date calculation functions, including day of the year, week of the year, and so on. Note that not all businesses consider ``week 1'' to be the same; for example, American businesses often consider the first week with a Monday in it to be Work Week #1, despite ISO 8601, which considers WW1 to be the first week with a Thursday in it.

How can I compare two dates and find the difference?

If you're storing your dates as epoch seconds then simply subtract one from the other. If you've got a structured date (distinct year, day, month, hour, minute, seconds values) then use one of the Date::Manip and Date::Calc modules from CPAN.

How can I take a string and turn it into epoch seconds?

If it's a regular enough string that it always has the same format, you can split it up and pass the parts to timelocal in the standard Time::Local module. Otherwise, you should look into the Date::Calc and Date::Manip modules from CPAN.

How can I find the Julian Day?

Neither Date::Manip nor Date::Calc deal with Julian days. Instead, there is an example of Julian date calculation that should help you in http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz ..

How do I find yesterday's date?

The time() function returns the current time in seconds since the epoch. Take one day off that:

$yesterday = time() - ( 24 * 60 * 60 );

Then you can pass this to localtime() and get the individual year, month, day, hour, minute, seconds values.

Does Perl have a year 2000 problem? Is Perl Y2K compliant?

Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is Y2K compliant (whatever that means). The programmers you've hired to use it, however, probably are not.

Long answer: The question belies a true understanding of the issue. Perl is just as Y2K compliant as your pencil--no more, and no less. Can you use your pencil to write a non-Y2K-compliant memo? Of course you can. Is that the pencil's fault? Of course it isn't.

The date and time functions supplied with perl (gmtime and localtime) supply adequate information to determine the year well beyond 2000 (2038 is when trouble strikes for 32-bit machines). The year returned by these functions when used in an array context is the year minus 1900. For years between 1910 and 1999 this happens to be a 2-digit decimal number. To avoid the year 2000 problem simply do not treat the year as a 2-digit number. It isn't.

When gmtime() and localtime() are used in scalar context they return a timestamp string that contains a fully-expanded year. For example, $timestamp = gmtime(1005613200) sets $timestamp to "Tue Nov 13 01:00:00 2001". There's no year 2000 problem here.

That doesn't mean that Perl can't be used to create non-Y2K compliant programs. It can. But so can your pencil. It's the fault of the user, not the language. At the risk of inflaming the NRA: ``Perl doesn't break Y2K, people do.'' See http://language.perl.com/news/y2k.html for a longer exposition.

Data: Strings

How do I validate input?

The answer to this question is usually a regular expression, perhaps with auxiliary logic. See the more specific questions (numbers, mail addresses, etc.) for details.

How do I unescape a string?

It depends just what you mean by ``escape''. URL escapes are dealt with in perlfaq9. Shell escapes with the backslash (\) character are removed with:

s/\\(.)/$1/g;

This won't expand "\n" or "\t" or any other special escapes.

How do I remove consecutive pairs of characters?

To turn "abbcccd" into "abccd":

s/(.)\1/$1/g;

How do I expand function calls in a string?

This is documented in perlref. In general, this is fraught with quoting and readability problems, but it is possible. To interpolate a subroutine call (in list context) into a string:

print "My sub returned @{[mysub(1,2,3)]} that time.\n";

If you prefer scalar context, similar chicanery is also useful for arbitrary expressions:

print "That yields ${\($n + 5)} widgets\n";

Version 5.004 of Perl had a bug that gave list context to the expression in ${...}, but this is fixed in version 5.005.

See also ``How can I expand variables in text strings?'' in this section of the FAQ.

How do I find matching/nesting anything?

This isn't something that can be done in one regular expression, no matter how complicated. To find something between two single characters, a pattern like /x([^x]*)x/ will get the intervening bits in $1. For multiple ones, then something more like /alpha(.*?)omega/ would be needed. But none of these deals with nested patterns, nor can they. For that you'll have to write a parser.

If you are serious about writing a parser, there are a number of modules or oddities that will make your life a lot easier. There is the CPAN module Parse::RecDescent, the standard module Text::Balanced, the byacc program, the CPAN module Parse::Yapp, and Mark-Jason Dominus's excellent py tool at http://www.plover.com/~mjd/perl/py/ ..

One simple destructive, inside-out approach that you might try is to pull out the smallest nesting parts one at a time:

while (s//BEGIN((?:(?!BEGIN)(?!END).)*)END/gs) {
# do something with $1
}

A more complicated and sneaky approach is to make Perl's regular expression engine do it for you. This is courtesy Dean Inada, and rather has the nature of an Obfuscated Perl Contest entry, but it really does work:

This is fine if you are just looking for a single character. However, if you are trying to count multiple character substrings within a larger string, tr/// won't work. What you can do is wrap a while() loop around a global pattern match. For example, let's count negative integers:

To force each word to be lower case, with the first letter upper case:

$line =~ s/(\w+)/\u\L$1/g;

You can (and probably should) enable locale awareness of those characters by placing a use locale pragma in your program. See perllocale for endless details on locales.

This is sometimes referred to as putting something into "title case", but that's not quite accurate. Consdier the proper capitalization of the movie Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb, for example.

How can I split a [character] delimited string except when inside [character]? (Comma-separated files)

Take the example case of trying to split a string that is comma-separated into its different fields. (We'll pretend you said comma-separated, not comma-delimited, which is different and almost never what you mean.) You can't use split(/,/) because you shouldn't split if the comma is inside quotes. For example, take a data line like this:

Due to the restriction of the quotes, this is a fairly complex problem. Thankfully, we have Jeffrey Friedl, author of a highly recommended book on regular expressions, to handle these for us. He suggests (assuming your string is contained in $text):

If you want to represent quotation marks inside a quotation-mark-delimited field, escape them with backslashes (eg, "like \"this\"". Unescaping them is a task addressed earlier in this section.

Alternatively, the Text::ParseWords module (part of the standard perl distribution) lets you say:

use Text::ParseWords;
@new = quotewords(",", 0, $text);

There's also a Text::CSV module on CPAN.

How do I strip blank space from the beginning/end of a string?

Although the simplest approach would seem to be:

$string =~ s/^\s*(.*?)\s*$/$1/;

This is unnecessarily slow, destructive, and fails with embedded newlines. It is much better faster to do this in two steps:

$string =~ s/^\s+//;
$string =~ s/\s+$//;

Or more nicely written as:

for ($string) {
s/^\s+//;
s/\s+$//;
}

This idiom takes advantage of the foreach loop's aliasing behavior to factor out common code. You can do this on several strings at once, or arrays, or even the values of a hash if you use a slide:

# trim whitespace in the scalar, the array,
# and all the values in the hash
foreach ($scalar, @array, @hash{keys %hash}) {
s/^\s+//;
s/\s+$//;
}

How do I pad a string with blanks or pad a number with zeroes?

(This answer contributed by Uri Guttman)

In the following examples, $pad_len is the length to which you wish to pad the string, $text or $num contains the string to be padded, and $pad_char contains the padding character. You can use a single character string constant instead of the $pad_char variable if you know what it is in advance.

The simplest method use the sprintf function. It can pad on the left or right with blanks and on the left with zeroes.

See also ``How do I expand function calls in a string?'' in this section of the FAQ.

What's wrong with always quoting "$vars"?

The problem is that those double-quotes force stringification, coercing numbers and references into strings, even when you don't want them to be. Think of it this way: double-quote expansion is used to produce new strings. If you already have a string, why do you need more?

If you get used to writing odd things like these:

print "$var"; # BAD
$new = "$old"; # BAD
somefunc("$var"); # BAD

You'll be in trouble. Those should (in 99.8% of the cases) be the simpler and more direct:

print $var;
$new = $old;
somefunc($var);

Otherwise, besides slowing you down, you're going to break code when the thing in the scalar is actually neither a string nor a number, but a reference:

You can also get into subtle problems on those few operations in Perl that actually do care about the difference between a string and a number, such as the magical ++ autoincrement operator or the syscall() function.

But the HERE_TARGET must still be flush against the margin. If you want that indented also, you'll have to quote in the indentation.

($quote = <<' FINIS') =~ s/^\s+//gm;
...we will have peace, when you and all your works have
perished--and the works of your dark master to whom you
would deliver us. You are a liar, Saruman, and a corrupter
of men's hearts. --Theoden in /usr/src/perl/taint.c
FINIS
$quote =~ s/\s*--/\n--/;

A nice general-purpose fixer-upper function for indented here documents follows. It expects to be called with a here document as its argument. It looks to see whether each line begins with a common substring, and if so, strips that off. Otherwise, it takes the amount of leading white space found on the first line and removes that much off each subsequent line.

Or with a fixed amount of leading white space, with remaining indentation correctly preserved:

$poem = fix<<EVER_ON_AND_ON;
Now far ahead the Road has gone,
And I must follow, if I can,
Pursuing it with eager feet,
Until it joins some larger way
Where many paths and errands meet.
And whither then? I cannot say.
--Bilbo in /usr/src/perl/pp_ctl.c
EVER_ON_AND_ON

Data: Arrays

What is the difference between a list and an array?

An array has a changeable length. A list does not. An array is something you can push or pop, while a list is a set of values. Some people make the distinction that a list is a value while an array is a variable. Subroutines are passed and return lists, you put things into list context, you initialize arrays with lists, and you foreach() across a list. @ variables are arrays, anonymous arrays are arrays, arrays in scalar context behave like the number of elements in them, subroutines access their arguments through the array @_, push/pop/shift only work on arrays.

As a side note, there's no such thing as a list in scalar context. When you say

$scalar = (2, 5, 7, 9);

you're using the comma operator in scalar context, so it evaluates the left hand side, then evaluates and returns the left hand side. This causes the last value to be returned: 9.

What is the difference between $array[1] and @array[1]?

The former is a scalar value, the latter an array slice, which makes it a list with one (scalar) value. You should use $ when you want a scalar value (most of the time) and @ when you want a list with one scalar value in it (very, very rarely; nearly never, in fact).

Sometimes it doesn't make a difference, but sometimes it does. For example, compare:

$good[0] = `some program that outputs several lines`;

with

@bad[0] = `same program that outputs several lines`;

The -w flag will warn you about these matters.

How can I extract just the unique elements of an array?

There are several possible ways, depending on whether the array is ordered and whether you wish to preserve the ordering.

a) If @in is sorted, and you want @out to be sorted: (this assumes all true values in the array)

$prev = 'nonesuch';
@out = grep($_ ne $prev && ($prev = $_), @in);

This is nice in that it doesn't use much extra memory, simulating uniq(1)'s behavior of removing only adjacent duplicates. It's less nice in that it won't work with false values like undef, 0, or ""; "0 but true" is ok, though.

How can I tell whether a list or array contains a certain element?

Hearing the word "in" is an indication that you probably should have used a hash, not a list or array, to store your data. Hashes are designed to answer this question quickly and efficiently. Arrays aren't.

That being said, there are several ways to approach this. If you are going to make this query many times over arbitrary string values, the fastest way is probably to invert the original array and keep an associative array lying about whose keys are the first array's values.

These are slow (checks every element even if the first matches), inefficient (same reason), and potentially buggy (what if there are regexp characters in $whatever?). If you're only testing once, then use:

How do I find the first array element for which a condition is true?

How do I handle linked lists?

In general, you usually don't need a linked list in Perl, since with regular arrays, you can push and pop or shift and unshift at either end, or you can use splice to add and/or remove arbitrary number of elements at arbitrary points. Both pop and shift are both O(1) operations on perl's dynamic arrays. In the absence of shifts and pops, push in general needs to reallocate on the order every log(N) times, and unshift will need to copy pointers each time.

If you really, really wanted, you could use structures as described in perldsc or perltoot and do just what the algorithm book tells you to do. For example, imagine a list node like this:

This is bad because splice is already O(N), and since you do it N times, you just invented a quadratic algorithm; that is, O(N**2). This does not scale, although Perl is so efficient that you probably won't notice this until you have rather largish arrays.

How do I sort an array by (anything)?

The default sort function is cmp, string comparison, which would sort (1, 2, 10) into (1, 10, 2). <=>, used above, is the numerical comparison operator.

If you have a complicated function needed to pull out the part you want to sort on, then don't do it inside the sort function. Pull it out first, because the sort BLOCK can be called many times for the same element. Here's an example of how to pull out the first word after the first number on each item, and then sort those words case-insensitively.

Why does defined() return true on empty arrays and hashes?

The short story is that you should probably only use defined on scalars or functions, not on aggregates (arrays and hashes). See "defined" in perlfunc in the 5.004 release or later of Perl for more detail.

Here we'll do a reverse numeric sort by value, and if two keys are identical, sort by length of key, and if that fails, by straight ASCII comparison of the keys (well, possibly modified by your locale -- see perllocale).

How can I always keep my hash sorted?

You can look into using the DB_File module and tie() using the $DB_BTREE hash bindings as documented in "In Memory Databases" in DB_File. The Tie::IxHash module from CPAN might also be instructive.

What's the difference between "delete" and "undef" with hashes?

Hashes are pairs of scalars: the first is the key, the second is the value. The key will be coerced to a string, although the value can be any kind of scalar: string, number, or reference. If a key $key is present in the array, exists($key) will return true. The value for a given key can be undef, in which case $array{$key} will be undef while $exists{$key} will return true. This corresponds to ($key, undef) being in the hash.

Why don't my tied hashes make the defined/exists distinction?

They may or may not implement the EXISTS() and DEFINED() methods differently. For example, there isn't the concept of undef with hashes that are tied to DBM* files. This means the true/false tables above will give different results when used on such a hash. It also means that exists and defined do the same thing with a DBM* file, and what they end up doing is not what they do with ordinary hashes.

How do I reset an each() operation part-way through?

Using keys %hash in scalar context returns the number of keys in the hash and resets the iterator associated with the hash. You may need to do this if you use last to exit a loop early so that when you re-enter it, the hash iterator has been reset.

How can I get the unique keys from two hashes?

First you extract the keys from the hashes into arrays, and then solve the uniquifying the array problem described above. For example:

How can I make my hash remember the order I put elements into it?

Why does passing a subroutine an undefined element in a hash create it?

If you say something like:

somefunc($hash{"nonesuch key here"});

Then that element "autovivifies"; that is, it springs into existence whether you store something there or not. That's because functions get scalars passed in by reference. If somefunc() modifies $_[0], it has to be ready to write it back into the caller's version.

This has been fixed as of perl5.004.

Normally, merely accessing a key's value for a nonexistent key does not cause that key to be forever there. This is different than awk's behavior.

How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?

References are documented in perlref and the upcoming perlreftut. Examples of complex data structures are given in perldsc and perllol. Examples of structures and object-oriented classes are in perltoot.

How can I use a reference as a hash key?

You can't do this directly, but you could use the standard Tie::Refhash module distributed with perl.

Data: Misc

How do I handle binary data correctly?

Perl is binary clean, so this shouldn't be a problem. For example, this works fine (assuming the files are found):

If you're on a POSIX system, Perl's supports the POSIX::strtod function. Its semantics are somewhat cumbersome, so here's a getnum wrapper function for more convenient access. This function takes a string and returns the number it found, or undef for input that isn't a C float. The is_numeric function is a front end to getnum if you just want to say, ``Is this a float?''

Or you could check out http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz instead. The POSIX module (part of the standard Perl distribution) provides the strtol and strtod for converting strings to double and longs, respectively.

How do I keep persistent data across program calls?

For some specific applications, you can use one of the DBM modules. See AnyDBM_File. More generically, you should consult the FreezeThaw, Storable, or Class::Eroot modules from CPAN. Here's one example using Storable's store and retrieve functions:

How do I print out or copy a recursive data structure?

The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great for printing out data structures. The Storable module, found on CPAN, provides a function called dclone that recursively copies its argument.

use Storable qw(dclone);
$r2 = dclone($r1);

Where $r1 can be a reference to any kind of data structure you'd like. It will be deeply copied. Because dclone takes and returns references, you'd have to add extra punctuation if you had a hash of arrays that you wanted to copy.

AUTHOR AND COPYRIGHT

When included as part of the Standard Version of Perl, or as part of its complete documentation whether printed or otherwise, this work may be distributed only under the terms of Perl's Artistic License. Any distribution of this file or derivatives thereof outside of that package require that special arrangements be made with copyright holder.

Irrespective of its distribution, all code examples in this file are hereby placed into the public domain. You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit would be courteous but is not required.